Gut microbiota-bile acid crosstalk contributes to intestinal damage after nitrate exposure in Bufo gargarizans tadpoles

Bile acids (BAs) are amphipathic steroid acids whose production and diversity depend on both host and microbial metabolism. Nitrate (NO₃^-) is a widespread pollutant in aquatic ecosystems, which can cause rapid changes in microbial community structure and function. However, the effect of gut microbiota reshaped by nitrate‑nitrogen (NO₃-N) on BAs profiles remains unclarified. To test this, intestinal targeted BAs metabolomics and fecal metagenomic sequencing were performed on Bufo gargarizans tadpoles treated with different concentrations of NO₃-N. NO₃-N exposure induced a reduction in the abundance of microbiota with bile acid-inducible enzymes (BAIs) and/or hydroxysteroid dehydrogenases (HSDHs), thus inhibiting the conversion of primary BAs to secondary BAs. Inhibition of BAs biotransformation decreased protective hydrophilic BAs (UDCA) and increased toxic hydrophobic BAs (CA and CDCA), which may contribute to intestinal histopathological damage. Moreover, we found that NO₃-N treatment increased microbial virulence factors and decreased Glycoside hydrolases, further highlighting the deleterious risk of NO₃-N. Overall, this study shed light on the complex interactions of NO₃-N, gut microbiota, and BAs, and emphasized the hazardous effects of NO₃-N pollution on the health of amphibians.